Method and apparatus for logging foam cement in cased boreholes

Abstract

An acoustic logging system that measures distribution of foam cement and other material in a casing-borehole annulus. The distribution is preferably displayed as map. A borehole tool utilizes at least one acoustic transducer with a known frequency response and mounted on a rotating scanning head that is pointed essentially perpendicular to the borehole wall. The response of the transducer is used to measure an original impedance map of material within the borehole-casing annulus. A fast Fourier transform is used to generate a FFT map from the data comprising the original impedance map. The original impedance map is then combined with the FFT map using logic rules to obtain a final impedance map that is indicative of material within the casing-borehole annulus.

1. A method for measuring material within a casing-borehole annulus, the method comprising:
 (a) obtaining a plurality of impedance measurements as a function of depth within said borehole; 
 (b) transforming said plurality of impedance measurements into frequency-domain samples using a FFT to obtain a plurality of FFT function values wherein each said FFT function value corresponds to a said impedance measurement at a specific depth; and 
 (c) at each said specific depth, combining said impedance measurement with said corresponding FFT function value using logic rules to obtain a final impedance value indicative of said material at that said specific depth. 
 
     
     
       2. The method of  claim 1  wherein said logic rules comprise:
 (a) defining an impedance range and a FFT function threshold; 
 (b) identifying the said material at said specific depth as conventional cement if a said impedance measurement at that depth is greater than an upper limit of said impedance range; 
 (c) identifying said material at said specific depth as foam cement if
 (i) a said impedance measurement at that depth is less that said upper limit and greater than a lower limit of said impedance range, and 
 (ii) said FFT function at that depth is greater than said FFT function threshold; 
 
 (d) identifying said material at said specific depth as liquid if
 (i) a said impedance measurement at that depth is less that said upper limit and greater than said lower limit of said impedance range, and 
 (ii) said FFT function at that depth is less than said FFT function threshold; and 
 
 (e) identifying the said material at said specific depth as gas if a said impedance measurement at that depth is less than said lower limit of said impedance range. 
 
     
     
       3. The method of  claim 2  wherein said upper limit of said impedance range is 2.3 Mrayls and said lower limit is 0.4 Mrayls. 
     
     
       4. A method for generating a map indicative of material within a casing-borehole annulus, the method comprising;
 (a) obtaining, within a plurality of borehole azimuthal arc segments, a plurality of impedance measurements as a function of depth within said borehole; 
 (b) for each said azimuthal arc segment, transforming said plurality of impedance measurements into frequency-domain samples using a FFT to obtain a plurality of FFT function values wherein each said FFT function value corresponds to a said impedance measurement at a specific depth; 
 (c) within each said azimuthal arc segment and at each said specific depth, combining said impedance measurement with said corresponding FFT function value using logic rules to obtain a final impedance value indicative of said material within that said specific arc segment and at that said specific depth; and 
 (d) displaying said impedance values as a function of depth and arc segment in which determined thereby generating said map. 
 
     
     
       5. The method of  claim 4  wherein said logic rules comprise:
 (a) defining an impedance range and a FFT function threshold; 
 (b) identifying the said material at said specific depth and said specific arc segment as conventional cement if a said impedance measurement is greater than an upper limit of said impedance range; 
 (c) identifying said material at said specific depth and said specific arc segment as foam cement if
 (i) a said impedance measurement is less that said upper limit and greater than a lower limit of said impedance range, and 
 (ii) said FFT function is greater than said FFT function threshold; 
 
 (d) identifying said material at said specific depth and said specific arc segment as liquid if
 (i) a said impedance measurement is less that said upper limit and greater than said lower limit of said impedance range, and 
 (ii) said FFT function is less than said FFT function threshold; and 
 
 (e) identifying the said material at said specific depth and said specific arc segment as gas if a said impedance measurement is less than said lower limit of said impedance range. 
 
     
     
       6. The method of  claim 5  wherein said upper limit of said impedance range is 2.3 Mrayls and said lower limit is 0.4 Mrayls. 
     
     
       7. The method of  claim 6  wherein the number of said arc segments is 72. 
     
     
       8. The method of  claim 6  wherein said plurality of arc segments encompasses a 360 degree sweep of said borehole. 
     
     
       9. Apparatus for measuring material within a casing-borehole annulus, the apparatus comprising:
 (a) a scanning head that rotates a scanning transducer assembly radially within a borehole; 
 (b) a mechanical subassembly for rotating said scanning head; 
 (c) a processor for recording and processing acoustic responses of said scanning transducer to obtain a plurality of impedance measurements within at least one azimuthal arc segment as a function of depth within said borehole; 
 (d) a relationship for transforming said plurality of impedance measurements into frequency-domain samples using a FFT to obtain a plurality of FFT function values wherein each said FFT function value corresponds to a said impedance measurement within said at least one azimuthal arc and at a specific depth; and 
 (f) logic rules for combining, at said at least one azimuthal arc segment and at said specific depth, said impedance measurement with said corresponding FFT function value using logic rules to obtain a final impedance value indicative of said material within that at least one azimuthal arc segment and at that said specific depth. 
 
     
     
       10. The apparatus of  claim 9  wherein said logic rules comprise:
 (a) defining an impedance range and a FFT function threshold; 
 (b) identifying the said material at said specific depth and within said at least one azimuthal arc segment as conventional cement if a said impedance measurement is greater than an upper limit of said impedance range; 
 (c) identifying said material at said specific depth and within said at least one azimuthal arc segment as foam cement if
 (i) a said impedance measurement is less that said upper limit and greater than a lower limit of said impedance range, and 
 (ii) said FFT function is greater than said FFT function threshold; 
 
 (d) identifying said material at said specific depth and within said at least one azimuthal arc segment as liquid if
 (i) a said impedance measurement is less that said upper limit and greater than said lower limit of said impedance range, and 
 (ii) said FFT function is less than said FFT function threshold; and 
 
 (e) identifying the said material at said specific depth and within said at least one azimuthal arc segment as gas if a said impedance measurement is less than said lower limit of said impedance range. 
 
     
     
       11. The apparatus of  claim 10  wherein
 (a) a plurality of impedance measurements is made within a plurality of said borehole azimuthal arc segments at that depth; 
 (b) said plurality of impedance measurements are transformed into frequency-domain samples using a FFT to obtain a plurality of FFT function values wherein each said FFT function value corresponds to a said impedance measurement at a specific depth and within a specific azimuthal arc segment; 
 (c) said plurality of impedance measurements are combined with said corresponding FFT function values using said logic rules to obtain a final impedance value indicative of said material within that specific arc segment and at that said specific depth; and 
 (d) said final impedance values are displayed as a function of depth and azimuthal arc segment in which determined thereby generating said map indicative of said material. 
 
     
     
       12. The apparatus of  claim 10  wherein said upper limit of said impedance range is 2.3 Mrayls and said lower limit is 0.4 Mrayls. 
     
     
       13. The apparatus of  claim 11  wherein the number of said arc segments is 72. 
     
     
       14. The apparatus of  claim 11  wherein said plurality of arc segments encompasses a 360 degree sweep of said borehole. 
     
     
       15. The apparatus of  claim 9  wherein said apparatus is conveyed along said borehole with a wireline.